1. Field of the Invention
The present invention relates to an electro theological fluid. More particularly, the present invention relates to a homogeneous electro theological fluid, the viscosity being able to be controlled by the application of an electric field.
2. Background Art
An electro rheological fluid has been known in the art as a liquid suspension comprising inorganic or polymeric particles dispersed in an electric insulating liquid. The liquid changes its viscosity rapidly and reversibly from a liquid to a plastic or a solid state when an electric field is applied. The phenomenon is called an electro rheological effect or the Winslow effect.
Particles of which the surface is polarized with ease when an electric field is applied are generally used as the foregoing dispersed particles. Examples of the dispersible inorganic particles include silica (U.S. Pat. No. 3,047,507, British Patent No. 1,076,754 and Japanese Patent Laid-Open Publication No.61-44998), and Zeolite (Japanese Patent Laid-Open Publication No. 62-95397).
Examples of the polymeric particles include an alginic acid, a glucose having carboxyl group, a glucose having sulphonic group (Japanese Patent Laid-Open Publication No. 51-33783), a divinylbenzene-crosslinked polyacrylic acid (Japanese Patent Laid-Open Publication No. 53-93186), and a resole-type phenolic resin (Japanese Patent Laid-Open Publication No. 58-179259).
Further, examples of the electric insulating liquids include hydrocarbon-type oils, silicone oils, ester-type oils, and fluorine-type oils.
Examples of the expected application field of electro rheological fluids are engine mounts, shock absorber, clutches, and the like.
However, a major problem associated with the foregoing electro rheological fluid has been the precipitation/separation of particles from the dispersed system. This fact is the main cause that prevents the conventional electro rheological fluid from being applied commercially.
Numerous attempts with limited success have been made to develop a homogeneous electro rheological fluid such as, for example, an electro rheological fluid prepared by use of a low-molecular weight liquid crystal such as methoxybenzylidene butylaniline [Japanese Journal of Applied Physics, 17, page 1525 (1978)]. Such a homogeneous fluid has very poor electro rheological effect.
In addition to the foregoing, Japanese Patent Publication No. 42-11315 discloses a liquid crystalline polymer-containing homogeneous solution having an electro rheological effect, and Japanese Patent Laid-Open Publication Nos. 4-191511 and 4-266997, and preparatory notes for the 16th forum on liquid crystal, page 82 (1990) describe a solution comprising a poly(.gamma.-benzyl L-glutamate), which is known as a lyotropic liquid crystalline polymer, dissolved in a low-boiling polar solvent such as dioxane, tetrahydrofuran, cresol; or a low-boiling chlorine-type solvent such as methylene chloride, and chloroform, wherein said solution can exert a marked electro rheological effect.
The foregoing electro rheological fluid prepared by use of the poly(.gamma.-benzyl L-glutamate) is a homogeneous system, and has a fairly good electro rheological effect, and therefore, the foregoing problems associated with the precipitation and separation of particles can be circumvented. The viscosity of the fluid, however, becomes high even before the application of an electric field and the difference in torque before and after the application of the electric field becomes small because the concentration of the poly(.gamma.-benzyl L-glutamate) is so high that the fluid exhibits liquid crystalline properties. As a result, the large difference in torque necessary for commercial application of the liquid cannot be obtained. In addition, contrary to general expectations, the viscosity may sometimes be decreased when an electric field is applied, thus, presenting a problem, e.g., in controlling viscosity.
The object of the present invention is to provide a homogeneous electro rheological fluid thereby circumventing the problem associated with the precipitation of particles, wherein the fluid can provide a large difference in torque before and after the application of an electric field.
The present inventors have intensively investigated to solve the above-described problems, and as a result, have found that the foregoing problem associated with the small difference in torque before and after the application of electric field can be circumvented by providing a homogeneous electro rheological fluid comprising a lyotropic liquid crystalline polymer dissolved in a solvent in low concentrations, said concentration being such that the fluid does not exhibit liquid crystalline properties. The present invention has been completed on the basis of this finding.